1. Field of the Invention
The present invention relates to an insulating material employed in electronic instruments having excellent heat transfer property, and good insulating property and electrical reliability under an high temperature, and a circuit substrate and a module in use thereof.
2. Discussion of Background
A metal base circuit substrate has conventionally been employed for mounting electronic parts. The metal base circuit substrate is composed by adhering a conductive metal foil on the surface of a metal substrate interposing an adhesive insulating layer. The insulating layer is normally composed by filling an inorganic filler such as alumina to an epoxy resin. The thermal conductivity of the insulating layer is not larger than 5.0.times.10.sup.-3 (cal/.degree.C..multidot.cm.multidot.sec). The glass transition temperature thereof is not larger than about 160.degree. C., since the glass transition temperature is determined by an organic material such as an epoxy resin. Further, a metal base circuit substrate has been known wherein either one of the thermal conductivity and the glass transition temperature of the insulating layer exceeds the above numerical values. However, there has been no metal base circuit substrate having a property wherein both of the thermal conductivity and the glass transition temperature exceeds these values.
For instance, Japanese Unexamined Patent Publication No. 286768/1990 discloses a circuit substrate wherein an inorganic filler can densely be filled by filling the inorganic filler having a specified particle size to an epoxy resin, an insulative adhesive composition agent for a circuit substrate having a high thermal conductivity is provided and a conductive metal foil is laminated on a metal substrate interposing the insulative adhesive composition.
Further, Japanese Examined Patent Publication No. 49920/1988 discloses that a substrate for a hybrid integrated circuit can be provided which has a sufficient adhesive strength and a high thermal conductivity by interposing an adhesive agent wherein an epoxy resin includes aluminum oxide and a silane species coupling agent and/or a titanate species coupling agent between a metal substrate and a conductive metal foil.
However, although the thermal conductivity of the conventional adhesive composition is good and the adhesive strength thereof is promoted, it has a problem as an insulating material employed for electronic instruments having excellent heat resistance, especially the insulating property and electric reliability under a high temperature. Accordingly, a ceramic substrate such as an alumina substrate instead of a metal base circuit substrate has been employed when an especially good heat resistance is required.
In the conventional adhesive composition, a bisphenol A type epoxy resin has been employed as an epoxy resin. However, the viscosity thereof is comparatively high and the glass transition temperature is about 100.degree. C. at most, since a bisphenol A type epoxy resin has normally been employed which is provided with the purity of approximately 80%.
In the mean time, the demands for "high-power" and high density mounting have more and more been increasing with respect to electric parts employed in an automobile, various industrial machines and instruments and electric appliances. In a power module mounted with elements such as these high-powered diode, transistors and integrated circuits, the heat transfer property sufficient for transferring the heat generated at the elements as well as high electric reliability under a high temperature environment are necessitated.
For instance, when a metal base circuit substrate is mounted with electronic parts, which is employed as a transistor module, a diode module or a solid state relay, the electric reliability under a high temperature as well as the sufficient heat transfer property are required since the temperature thereof is equal to or more than 125.degree. C. and a high voltage of AC 300 V or more is applied thereon as an input voltage. Further, a high laminating strength of a copper foil under an elevated temperature is required for a power module which is employed in a place in the vicinity of an engine of an automobile (hereinafter, engine room), since heat is generated by the engine and the temperature of the power module becomes elevated, although the voltage thereof is comparatively low.
However, the glass transition temperature is low and the viscosity is comparatively high in the conventional bisphenol A type epoxy resin having a normal purity of about 80%. Accordingly, an insulating layer having a high thermal conductivity can not be formed since an inorganic filler material can not be filled in a large quantity and a filler material having a high thermal conductivity has not been employed. As stated above, there has been no insulating layer which satisfies simultaneously the thermal conductivity of 5.0.times.10.sup.-3 (cal/.degree.C..multidot.cm.multidot.sec) or more and the glass transition temperature of 164.degree. C. or more. Therefore, the conventional metal base circuit substrate can not be employed under these severe environments since the electric reliability thereof is poor. Further, in case of a ceramic substrate, a large substrate is difficult to be formed and the substrate is liable to cause cracks by impact or the like.